Arrangement for disconnecting branches of a low voltage supply network under short circuit conditions

ABSTRACT

A single circuit-breaker is provided in the phase conductor, and the bridging circuit is provided on the load side of the single circuit-breaker, and a magnetically driven switching contact closes a bridging current branch leading to the neutral conductor. The bridging circuit conducts a part of the short-circuit current past the magnetic drive and/or the switching contact. A resistor whose resistance value amounts to between 25 and 50% of the value of the smallest load-circuit impedance (L 2 , R 2 ) to be expected, may be provided in the bridging current branch. In such an arrangement, the bridging circuit and the circuit-breaker can form one unit.

BACKGROUND OF THE INVENTION

The present invention relates generally to apparatuses for disconnectingbranches of a low voltage supply network under short circuit conditions,and more particularly to an apparatus for disconnecting branches of alow voltage supply network under short circuit conditions, in whichpower consumers are connected in the branch to phase conductors and to acorresponding neutral conductor, a switching element is connected in theincoming circuit to the consumer in order to switch it on and/or off,and switching elements for short-circuit breaking being provided in thephase conductor and/or the neutral conductor, to which switchingelements a bridging circuit with a switching contact is connected, andin which a single circuit-breaker is also provided in the phaseconductor.

Usually current-limiting circuit-breakers or line safety switches areused for the short-circuit protection of low voltage branches. Theswitching devices limit the pass-through currents to typically 1/3 to1/2 of the current amplitude of the prospective short-circuit current.However, sensitive electrical devices, such as contactors or relays, arenot able to tolerate the short-circuit effects of such high pass-throughcurrents. Thus, grounding contacts or relay contacts may opendynamically in response to electrodynamic forces at high currents andbecome welded together when they close again as a result of contactfusing caused by arcs.

Various possibilities for limiting the effects of short circuits on theload side of protective devices by bridging a low-voltage branch arealready well known in the art. For example, German A-40 40 359 describesa hybrid bridging device, which limits the Joulean heat integral toabout 1/20 of the value of current-limiting line safety switches on theload side of the protective circuit. This very high protective action isnecessary when using semiconductor switches in low-voltage branches, forexample to control consumers, since overload currents of a shortduration must not exceed ten times the value of the rated current inorder not to destroy the semiconductor. However, this requiresconsiderable outlay for equipment, since the hybrid bridging deviceneeds a semiconductor switch with a higher rated current and acurrent-detecting and driving device. In addition, EP-A-0 504 463proposes bridging low-voltage branches, where the circuit-breaker isprovided with a tap of the arc chute and the switching arc starts up thebridging. In this case, the short-circuit protection depends on theoperating speed of the switch mechanism and on the transit speed of theswitching arc. Therefore, only a relatively small reduction in theJoulean heat integral is achieved on the load side of the protectivedevice, which amounts more or less to half of the pass-through Jouleanheat integral.

Furthermore, German A-41 10 335 discloses a device for short-circuitprotection, in which is connected a semiconductor switch in incomingcircuit to the sensitive consumer, and the semiconductor switchconnected in series to a mechanical switch and contains a bridgingbranch circuit leading to the neutral conductor. The bridging branchcircuit is thereby provided with a bridging switch, which is switched oninstantaneously by the short-circuit current and bridges the consumerand an additional circuit-breaker. By this means, the consumer and thesensitive semiconductor switch are removed from load, it being necessaryin any case for line safety switches to be present in the phaseconductor and in the neutral conductor, which entails additionalexpenditure.

In addition, U.S. Pat. No. 2,924,752 discloses an arrangement fordisconnecting three-phase low-voltage supply networks using a three-polecircuit breaker, in which a bridging circuit with a bridging currentbranch is provided, by way of which a part of the short-circuit currentis conducted past the magnetic drive via switching contacts. Due to thespecial design of the bridging current branch, this arrangement isintended exclusively for multipole networks.

The present invention is therefore directed to an apparatus forproviding short-circuit protection, which will greatly limit theshort-circuit current and the short-circuit duration in sensitiveelectrical devices, and which will have a simpler construction.

SUMMARY OF THE INVENTION

The present invention solves this problem by providing only one singlecircuit-breaker in the phase conductor, and by disposing the bridgingcircuit on the load side of the single circuit-breaker, and by providinga magnetically driven switching contact closing a bridging currentbranch leading to the neutral conductor. The bridging current branch hasa resistor whose resistance value amounts to between 25% and 50% of thevalue of the smallest load-circuit impedance to be expected. Preferably,the bridging circuit conducts part of the short-circuit current throughthe switching contact.

Thus, the arrangement according to the present invention is simplifiedcompared to existing prior art arrangements. A bridging contact ispresent, through which the short-circuiting current is directed to theneutral conductor, without an additional switch being required; theresistor, whose resistance value amounts to about 1/4 to 1/2 of thevalue of the smallest load-circuit impedance to be expected, beingprovided to limit the short-circuiting current in the bridging branch.The circuit-breaker and the bridging branch can advantageously form oneunit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a circuit diagram of an example of a bridging circuitaccording to an embodiment of the present invention.

FIG. 2 depicts a circuit diagram of a common protection device formed byintegrating the circuit-breaker and the bridging device.

FIG. 3 shows a circuit diagram of another embodiment of FIG. 2illustrating different connecting points for the slide rails.

FIG. 4 shows a circuit diagram of another embodiment of FIG. 2 in whichthe magnetic drives are configured in the load circuit.

FIG. 5 depicts a circuit diagram of another embodiment of FIG. 3 inwhich the magnetic drives are configured in the load circuit.

DETAILED DESCRIPTION

FIG. 1 shows a single-phase electrical network having an impedance L₁,R₁ on the incoming supply side of a circuit-breaker 1, which, on theload side, has a bridging circuit 10, as well as a short-circuitimpedance L₂, R₂, which leads to a sensitive system part formed by acontactor 8 and a consumer 9, e.g., a motor.

Connected up on the load side of the circuit-breaker 1 comprising theconnecting points A and B, is a bridging circuit 10, which has a switch11 constituting the actual bridging contact, a corresponding magneticdrive 12, and a bridging current branch 13 comprising the resistor 14.In this case, the bridging current branch 13 is connected to point D,the bridging circuit 10 has the connecting point C leading to thecontactor 8, and has the connecting point D connected to the neutralconductor.

A circuit arrangement of this type can also be designed for multiphasewiring systems, so that a multipole monitoring circuit is used.

Based on the example of a line protection for 16 A rated current,considering the configuration of FIG. 1, the magnetic drive of thebridging device can be so rated that, given dead short-circuits ofI_(prosp). of 3 to 6 kA, response times of about 1.2 to 0.8 ms arereached for the magnetic drive. The rapid removal of load from the loadcircuit by means of the bridging circuit 10 leads in this case to a I².tload, which now amounts to about 1/5 to 1/10 of the pass-through I₂.tvalue of the line safety switch.

In FIGS. 2 through 5, the circuit-breaker 1 is comprised of an arc chutewith arc chute plates 3 and slide rails 4, a corresponding switch 5, athermal trip element 6 and a magnetic drive 7. The broken line indicatesthe common protective device formed by integrating the circuit-breaker 1and the bridging device 10.

In FIG. 2, after the bridging current branch is brought into circuit,the magnetic coil 7 of the trip element 6 and the magnetic coil 12 ofthe bridging device 10 continue to carry current until the short-circuitcurrent on the power supply side becomes zero. A current separationcorresponding to the impedance ratio of R₃ to R₂, L₂ takes place betweenthe bridging current branch and the load circuit. At very large timeconstants L₂ /(R₂ +R₃), a load current can still flow, although thesupply-side short-circuit current is interrupted. In this case, thebridging contact 11 opens and the switching voltage accelerates thedecay of the load current.

FIG. 3 indicates another embodiment of FIG. 2 in which the connectingpoints of the slide rails 4 permit the arc chute 2 to bridge thecircuit-breaker contact 5, the thermal trip element 6, and the magnetictrip element 7, as well as the magnetic drive 12 and the correspondingswitch 11. After commutation of the arc into the arc chute 2, themagnetic drive 12 of the bridging device 10, inter alia, becomescurrentless and the switch 11, with its switching voltage, interruptsthe load current relatively early.

In the embodiment illustrated in FIG. 2, the switching contact of theswitch 11 normally switches free of arcs. However, in the bridgingdevice of FIG. 3, an arc loading occurs at the switching contact ofswitch 11, which has to be taken into consideration when designing thecontact members used here.

FIGS. 4 and 5 illustrate an embodiment in which the protective devicesdiffer from those of FIGS. 2 and 3 essentially in that the magneticdrives 7 and 12 are configured in the load circuit instead of in thesupply circuit. To this end, the magnetic drives 7 and 12 no longer needto be rated for full short-circuit loading.

For that reason, the arrangement according to FIG. 5 is especiallyadvantageous, because after load is removed from the switch 11 as theresult of electric arc commutation into the arc chute 2, its switchingcontact remains closed for as long as the magnetic excitation of themagnetic drives 7 and 12 persists due to the load current. Therefore,the switch 11 opens the switching contact free of electric arcs afterthe load current has decayed.

The examples depicted in FIGS. 2 through 5 can be modified in variousdetails.

For example, the arrangement of the thermal trip element 6 can be placedbetween the terminal connection C and the connecting point of thebridging branch. The magnetic drives 7 and 12 can be replaced by acommon magnetic drive for tripping and bridging. When the intent is forthe bridging circuit to have a multipole design, the individualconnections are connected to one another, respectively, and to theneutral conductor. In the case of an integrated structure as a multipolebridging switch, the connection of the terminal connections D take placein the unit.

In the case of the described arrangement, it is advantageous thatcontact members of weld-resistant contact materials are used for theswitching contact of the switch 11. Possible materials for this arematerials on a silver metal-oxide (AgMeO) base, such as the known AgSnO₂8, or silver- or copper-graphite materials, such as, for example, AgC5or CuC7. An asymmetrical contact pairing with different materials isalso possible.

What is claimed is:
 1. An apparatus for disconnecting branches of a lowvoltage supply network under short circuit conditions, wherein consumersare coupled in one branch to a phase conductor and to a correspondingneutral conductor, the apparatus comprising:a switching element coupledin an incoming circuit to the consumer in order to switch the switchingelement to at least one of an on state and an off state; a singlecircuit-breaker disposed in the phase conductor; and a bridging circuitpositioned on a load side of the single circuit-breaker and disposed inat least one of the phase conductor and the neutral conductor, thebridging circuit including a first magnetic drive and a bridging switch,the bridging switch having a magnetically driven switching contact forclosing a bridging current branch leading to the neutral conductor, thebridging current branch including a resistor having a resistance valuebetween 25% and 50% of a value of a smallest expected load-circuitimpedance, wherein the bridging current branch conducts a part of ashort-circuiting current through the magnetically driven switchingcontact in the bridging circuit so that the magnetically drivenswitching contact leading to the neutral conductor is closed.
 2. Anapparatus for disconnecting branches of a low voltage supply networkunder short circuit conditions, wherein consumers are coupled in onebranch to a phase conductor and to a corresponding neutral conductor,the apparatus comprising:a switching element coupled in an incomingcircuit to the consumer in order to switch the switching element to atleast one of an on state and an off state; a single circuit-breakerdisposed in the phase conductor; and a bridging circuit positioned on aload side of the single circuit-breaker and disposed in at least one ofthe phase conductor and the neutral conductor, the bridging circuitincluding a first magnetic drive and a bridging switch, the bridgingswitch having a magnetically driven switching contact for closing abridging current branch leading to the neutral conductor, the bridgingcurrent branch including a resistor having a resistance value between25% and 50% of a value of a smallest expected load-circuit impedance,wherein the bridging current branch conducts a part of ashort-circuiting current through the magnetically driven switchingcontact in the bridging circuit so that the magnetically drivenswitching contact leading to the neutral conductor is closed, andwherein the circuit-breaker and the bridging circuit form one unit. 3.The apparatus according to claim 2, further comprising a second magneticdrive for the circuit-breaker, wherein both the first and secondmagnetic drives are situated in the load circuit.
 4. The apparatusaccording to claim 2, wherein a common magnetic drive is provided fortripping the circuit-breaker and for switching on the bridging switch.5. The apparatus according to claim 2, wherein the circuit-breakerfurther comprises an arc chute having slide rails, wherein the bridgingcurrent branch is coupled to the slide rails so that the thermal and themagnetic trip elements and the first magnetic drive are bridged via thearc chute of the circuit-breaker.
 6. An apparatus for disconnectingbranches of a low voltage supply network under short circuit conditions,wherein consumers are coupled in one branch to a phase conductor and toa corresponding neutral conductor, the apparatus comprising:a switchingelement coupled in an incoming circuit to the consumer in order toswitch the switching element to at least one of an on state and an offstate; a single circuit-breaker disposed in the phase conductor; abridging circuit positioned on a load side of the single circuit-breakerand disposed in at least one of the phase conductor and the neutralconductor, the bridging circuit including a first magnetic drive and abridging switch, the bridging switch having a magnetically drivenswitching contact for closing a bridging current branch leading to theneutral conductor, the bridging current branch includes a resistorhaving a resistance value between 25% and 50% of a value of a smallestexpected load-circuit impedance; and a thermal trip element for thecircuit-breaker, the thermal trip element being disposed on the loadside of the bridging current branch, wherein the bridging current branchconducts a part of a short-circuiting current through the magneticallydriven switching contact in the bridging circuit so that themagnetically driven switching contact leading to the neutral conductoris closed.
 7. The apparatus according to claim 6, wherein thecircuit-breaker further comprises an arc chute having slide rails,wherein the bridging current branch is coupled to the slide rails sothat the thermal and the magnetic trip elements and the magnetic driveare bridged via the arc chute of the circuit-breaker.
 8. An apparatusfor disconnecting branches of a low voltage supply network under shortcircuit conditions, wherein consumers are coupled in one branch to aphase conductor and to a corresponding neutral conductor, the apparatuscomprising:a switching element coupled in an incoming circuit to theconsumer in order to switch the switching element to at least one of anon state and an off state; a single circuit-breaker disposed in thephase conductor; and a bridging circuit positioned on a load side of thesingle circuit-breaker and being disposed in at least one of the phaseconductor and the neutral conductor, the bridging circuit including afirst magnetic drive and a bridging switch, the bridging switch having amagnetically driven switching contact for closing a bridging currentbranch leading to the neutral conductor, the bridging current branchincluding a resistor having a resistance value between 25% and 50% of avalue of a smallest expected load-circuit impedance, wherein thebridging current branch conducts a part of a short-circuiting currentthrough the magnetically driven switching contact in the bridgingcircuit so that the magnetically driven switching contact leading to theneutral conductor is closed, and wherein the switching contact of themagnetically driven switch of the bridge circuit is made ofweld-resistant contact materials.
 9. The apparatus according to claim 8,further comprising a plurality of connectors for coupling to the lowvoltage supply network, wherein said plurality of connectors areasymmetrically paired.
 10. The apparatus according to claim 8, whereinthe base for the weld-resistant contact materials is selected from thegroup of materials consisting of silver metal-oxide (AgMeO),silver-graphite (AgC) and copper-graphite (CuC).
 11. The apparatusaccording to claim 10, further comprising a plurality of connectors forcoupling to the low voltage supply network, wherein said plurality ofconnectors are asymmetrically paired.
 12. A multipole bridging apparatusfor multi-phase networks under short circuit conditions, whereinconsumers are coupled in the branch to a phase conductor and to acorresponding neutral conductor, the multipole bridging apparatuscomprising:a switching element coupled in an incoming circuit to theconsumer in order to switch the switching element to at least one of anon state and an off state; a single multipole circuit-breaker disposedin the phase conductor; and a bridging circuit positioned on a load sideof the single circuit-breaker and being disposed in at least one of thephase conductor and the neutral conductor, the bridging circuitincluding a first magnetic drive and a bridging switch, the bridgingswitch having a magnetically driven switching contact for closing abridging current branch leading to the neutral conductor, the bridgingcurrent branch including a resistor having a resistance value between25% and 50% of a value of a smallest expected load-circuit impedancewherein the bridging current branch conducts a part of ashort-circuiting current through the magnetically driven switchingcontact in the bridging circuit so that the magnetically drivenswitching contact leading to at least one of a common point of thebridging current branch and the neutral conductor is closed.